The invention relates generally to nozzles, and more particularly to a variable geometry nozzle for turbines in a turbocharger, for example.
Fixed geometry turbochargers are normally designed for a specific operating condition of the internal combustion engine. Hence, such a turbocharger may be in an off-design condition across a wide range of engine operating points. Performance of a fixed geometry turbocharger may be reduced during part load operation and hence, may introduce challenges associated with meeting the air handling requirements for emissions and performance. Variable geometry turbochargers may be used to address problems associated with the fixed geometry turbochargers. The optimal turbine requirements for part load conditions are very different from the optimal requirements at full load. Variable geometry turbochargers (VGT) address such a limitation by allowing the effective size of the turbine nozzle to vary from part load to full load operating conditions.
The effective turbine size is determined primarily by a turbine nozzle throat area. A variable nozzle turbine alters the effective nozzle throat area and turning angle in order to effect a change in the turbine operating characteristics. The nozzle throat area is varied depending on an engine load. The variable nozzle turbine provides transient response benefits and specific fuel consumption benefits during both full load and partial load conditions. The variable nozzle turbine also allows customizing the air handling system for different operating conditions.
Conventional variable geometry turbochargers are implemented with rotatable/sliding nozzle guide vanes. In one conventional implementation, each nozzle airfoil is a separate piece that rotates about its own axis. Each nozzle airfoil requires a bushing/bearing/seal and requires a mechanically complex actuating mechanism. The large number of components, seals, and moving parts make the conventional VGT systems unattractive from the point of view of cost and reliability.